I could easily be mistaken on this, but I was under the impression the sidekicks in the toy box games were townspeople-sized, like the superhero helpers in the Incredibles playset. Granted, my memory of this is from early on when the toy box games were first talked about and I could be remembering things wrong. If I
have remembered that correctly, then Carol's not a sidekick from the toy box games. While all the wave 1 Marvel power discs have been announced, it is possible there's a second wave of them, and she could be a sidekick disc. But I definitely have my fingers crossed that she's playable - she has a hugely devoted fanbase, so there's a very willing audience out there for her as a figure. Putting a character in a combat situation in a teaser trailer only to have them turn out to be an NPC just seems mean!
I know the first post of this thread reports she's a mission giver in the Avengers playset, but if Loki can be playable in the playset despite being the villain, I don't see why Carol can't. Plus I've heard a lot of talk about Carol coming to the Marvel Cinematic Universe - it's strongly rumoured she'll be appearing in Jessica Jones' Netflix series. So between that and her appearances in Earth's Mightiest Heroes, she has appeal to a wider audience than just comic readers. Heres hoping!